1. Field of the Invention
The present disclosure relates to an analog-to-digital (A/D) converter, a photoelectric conversion device, and an imaging system.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2008-259228 discusses a photoelectric conversion device used in digital cameras, digital camcorders, and the like. The photoelectric conversion device includes a column analog-to-digital converter (ADC), as one of exemplary systems thereof, in which each column of a pixel array includes an ADC. Among the systems used in the column ADC, a ramp type ADC is widely used. In the ramp type ADC, an analog signal and a reference signal which varies with time are compared in magnitude to each other in order to measure a time period after the reference signal starts varying before the magnitude relation therebetween is reversed.
In the ramp type ADC, the time period after the reference signal starts varying before the magnitude relation between the reference signal and the analog signal is reversed is measured by using a counter circuit. The counter circuit is controlled as to whether the counter circuit counts the clock signal by a count actuating signal. In other words, where the counter circuit receives a clock signal and a count actuating signal, however, if timing for inputting both signals is not managed, a count start time and a count completion time may be shifted from target timing upon performing the analog-to-digital (A/D) conversion. More specifically, in the photoelectric conversion device, even signals generated based on the same amount of incident light may be converted into different digital values. As a result, an image obtained therefrom may be degraded.